RF Tags can be used in a multiplicity of ways for locating and identifying accompanying objects, items, animals, and people, whether these objects, items, animals, and people are stationary or mobile, and transmitting information about the state of the of the objects, items, animals, and people. It has been known since the early 60's in U.S. Pat. No. 3,098,971 by R. M. Richardson, that electronic components on a transponder could be powered by radio frequency (RF) power sent by a "base station" at a carrier frequency and received by an antenna on the tag. The signal picked up by the tag antenna induces an alternating current in the antenna which can be rectified by an RF diode and the rectified current can be used for a power supply for the electronic components. The tag antenna loading is changed by something that was to be measured, for example a microphone resistance in the cited patent. The oscillating current induced in the tag antenna from the incoming RF energy would thus be changed, and the change in the oscillating current led to a change in the RF power radiated from the tag antenna. This change in the radiated power from the tag antenna could be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. In the cited patent, the antenna current also oscillates at a harmonic of the carrier frequency because the diode current contains a doubled frequency component, and this frequency can be picked up and sorted out from the carrier frequency much more easily than if it were merely reflected. Since this type of tag carries no power supply of its own, it is called a "passive" tag to distinguish it from an active tag containing a battery. The battery supplies energy to run the active tag electronics, but not to broadcast the information from the tag antenna. An active tag also changes the loading on the tag antenna for the purpose of transmitting information to the base station.
The "rebroadcast" or "reflection" of the incoming RF energy at the carrier frequency is conventionally called "back scattering", even though the tag broadcasts the energy in a pattern determined solely by the tag antenna and most of the energy may not be directed "back" to the transmitting antenna.
In the 70's, suggestions to use tags with logic and read/write memories were made. In this way, the tag could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal. The antenna load was changed by use of a transistor. A transistor switch also changed the loading of the transponder in U.S. Pat. No. 4,786,907 by A. Koelle.
Prior art tags have used electronic logic and memory circuits and receiver circuits and modulator circuits for receiving information from the base station and for sending information from the tag to the base station.
The continuing march of semiconductor technology to smaller, faster, and less power hungry has allowed enormous increases of function and enormous drop of cost of such tags. Presently available research and development technology will also allow new function and different products in communications technology.
Prior art tags which use a different frequency than that of the base station carrier frequency are disadvantageous in that the tag and base station antennas usually have maximum efficiency if it they are designed for a single frequency.
Prior art battery tags which use the same frequency as that of the base station must modulate the antenna reflectance with a well defined modulation frequency so that the base station can distinguish the modulated reflected signal from the various sources of noise. Prior art battery tags carry an oscillator as part of the circuitry needed to receive and send data between the tag and the base station. This oscillator needs a local frequency standard to fix the tag modulation frequency so that the base station can easily and cheaply receive and demodulate the modulated signal sent by the tag. Such local frequency standards are very expensive and hard to integrate on a monolithic semiconductor chip. The tags require an oscillator which draws a lot of current from the tag power supply, either from a battery tag or a passive tag, which lowers either the life of the battery or the range of the tag, respectively. In addition, the base station may have to adjust to the modulation frequency sent out by the tag, which requires that the base station listens to the tag in the first step of the communication procedure, instead of talking to the tag first. This complicates communication procedures when there are multiple tags in the field. Each tag may be sending signals to the base station with a different modulation frequency, and the signals will interfere.